It hardly seems likely that a novel could contain too much understanding. Isn't that what fiction aims at, supersaturation with insight? Still, that's the strongest criticism that can be made against Anne Tyler's new novel, a return to form by a great writer who has recently (with Back When We Were Grownups and, especially, The Amateur Marriage) seemed to be overworking an established vein.

Digging to America starts at Baltimore airport in August 1997. The Donaldsons, a large family armed with video cameras and tape recorders, are awaiting the arrival of a Korean baby girl. International adoption is already rich territory, representing baby hunger at its most extreme (or human trafficking at its most benign, if you prefer), but then the point of view slides past the Donaldsons and rests on the Yazdans, a family of Iranian descent who are expecting their own orphan, with much less fanfare.

The two families keep in touch, thanks to the warm interferingness of Bitsy Donaldson, though their approaches to non-biological parenthood differ from the start. Ziba and Sami Yazdan immediately Americanise their daughter's name from Sooki to Susan, while the Donaldsons continue to call theirs Jin-Ho and to dress her in Korean clothes.

For the Iranians, American identity isn't something to be taken for granted - it takes work. There's comedy in the way unorthodox arrangements reproduce the flaws of the standard model. In 1997, an arrival party happens almost by itself, but soon it becomes established as a duty, requiring more and more elaborate preparation and creating its own rebels against the festive spirit.

Anne Tyler has always excelled at revealing the complex grain of what appears at first sight to be white-bread America, but here, she is after something different. It takes great confidence to appropriate a community's jokes about itself; to write of a Donaldson attending a traditional New Year party (held in March) that he was 'almost the only American present. Oh, a few of the young male cousins had married blondes - there was no getting past that Iranian thing about blondes - but still the man was noticeable for his pale-skinned, faded appearance.'

Widowed Maryam, Susan's new grandmother, grew up in Tehran but emigrated in the days of the shah, leaving for America in something that looked like an arranged marriage, with a doctor already established in Baltimore, but was more of a happy accident. There is no end to the nuance of assimilation and role-playing. Maryam's closest cousin, for instance, based in Vermont, dresses in a souped-up bazaar style that would be way over the top in Tehran. Her American husband cooks Iranian dishes which he announces by their proper names. On one occasion, when he offers her a second helping of polo, Maryam can't help snapping: 'Why don't you just call it rice?'

She assumes that any American who showed an interest in her would be typecasting her along the same lines, as Madame Iran. At one point, Maryam wears an old T-shirt bearing the slogan 'Foreigner ', which is assumed by a Donaldson to refer to the rock group. But no, it means what it says. Sami jokingly had it printed for her to cheer her up on the day she became an American citizen. She was happy about her new status, but sad as well - she calls it the Immigration Tango.

Tyler's writing is light in texture, but very filling. In one quietly devastating passage, from the point of view of a man recently bereaved, the fate of a tree recapitulates his wife's cancer: 'He sat dully at the kitchen table and gazed out at the neighbours' backyard, where the tree men were cutting down a huge old gnarly maple. The day before they had lopped off the leafy tip ends and fed them to the chipper, and he could imagine that overnight the maple must have stood there in some botanical version of shock. But only the smallest branches had been removed, after all. A tree so large could adjust to that. This morning, though, the men had moved on to the larger branches and perhaps that, too, could have been adjusted to even though the tree had become as stubby and short-armed as a saguaro cactus. But now they were setting their chainsaws to work on the trunk itself, and all those earlier adjustments turned out to have been for nothing.'

Point of view is passed on from chapter to chapter in a subtle dance. This is beautifully done, but the effect of multiple viewpoints is to muffle the distinctiveness of the first. It's not quite that each excavation of a character's world shovels earth into the hole left by the last, more that there is an impoverishment of shadows when we as readers know more than any single person in the story. A novel needs such shadows as much as any painting.

In particular, it feels rude to know more than Maryam does. The novel belongs to her, even if she is polite enough to give way to others. Tyler gives her the first point of view and makes her the first character to have a second helping (and the only one to have a third). Maryam is a triumph of characterisation not because she is an exotic and so, by implication, a stretch for the author, but because she is by instinct and habit so elusive, never so much herself as when she withholds herself. She is empathetic and tactful, but not social. Her dignity is perhaps her weak spot. When she puts on a bicycle helmet and then can't remove it, so that she must visit her son and daughter-in-law for help, the sense of cutting a comic figure, even in front of intimates, is quiet torture for her.

Maryam resists any temptation to live in the past. When her Vermont-based cousin reminisces about the past they shared, Maryam hardly recognises it. 'All her memories seemed to involve hilarious parties, or wagon rides at the family's summer place in Meigun, or day-long picnics with every single relative on both sides in attendance. Where were the quarrels and the schisms, the uncle who took opium and the uncle who embezzled, the aunts' endless, bitter competition for their father's grudging notice? Did Farah not remember the cousin who killed herself when they forbade her to go to medical school, or the cousin who was refused permission to marry the boy she loved?'

Maryam works (a few days a week at a pre-school) but is hardly career-minded. At work, she is meticulous about technology, emptying her computer's trash at the end of every day and defragmenting the hard drive exactly once a month, but personally she doesn't bother with a mobile phone or even an answering machine, on the basis that if you want to speak to her you're likely to try again. Even at a busy social occasion like an arrival party, she polices her own gestures - '"Well, of course," Maryam said, and she set her cup back in her saucer without a sound.' That missing chink is the keynote of a magnificent piece of character-drawing.